Interpretive Summary: Cattle are naturally infected with E. coli O157:H7, a bacteria that causes food sickness in humans. This bacteria is more common in cattle in the summer months than in the winter, but nobody knows the reason for this seasonality. Vitamin D provided by natural sunlight increases the concentrations of vitamin D in cattle during the summer months. We speculated that this increase in vitamin D may play a role in the seasonality of E. coli O157:H7 and conducted three experiments to examine its potential role. Results support the idea that vitamin D may play a limited role in the seasonality of this pathogen; however, they also indicate that making changes in the amount of vitamin D fed to cattle may not be substantial enough to affect changes on E. coli O157:H7.

Technical Abstract:
Previous research conducted in our laboratory demonstrated that the seasonal shedding pattern of E. coli O157:H7 in cattle is related to physiological responses within the animal to changing day-length. In a continuation of this research, we examined the effect of vitamin D supplementation in cattle naturally colonized with E. coli O157:H7. Experiment I was conducted in the fall of the year with the hypothesis that cattle administered exogenous vitamin D would continue to shed E. coli O157:H7, whereas a seasonal decrease in fecal shedding would be observed in control animals. Fourteen crossbred beef calves (avg. BW 225 kg) and 12 Holstein steers (avg. BW 454 kg) were randomly assigned to one of two treatments, with each cattle type equally represented in both treatments: control (empty gelatin capsule) or 0.5 x 106 IU vitamin D administered daily via oral bolus for ten days. All animals were maintained in an outdoor dry-lot pen and fed a high concentrate ration. Fecal samples were collected from individual animals via rectal palpation daily [6 d prior to (phase 1) and throughout the 10 d treatment administration (phase 2)] for culture of E. coli O157:H7. Blood samples were collected for determination of serum vitamin D concentrations one day following the final vitamin D dose. No differences in the percentage of Holsteins or beef calves shedding E. coli O157:H7 were observed during phase 1. However, during phase 2, more calves in the vitamin D treatment tended (P = 0.11) to shed E. coli O157:H7 compared to controls (6.5 versus 14.3% for control and vitamin D treatments, respectively). The percentage of Holsteins shedding E. coli O157:H7 tended (P = 0.12) to be higher in the control (34.9%) compared to the vitamin D (22.7%) treatment but decreased in both treatments in relation to the decreasing day length. Serum concentrations of vitamin D were markedly higher (P < 0.0001) in treated (782 nMol/L) versus control (258 nMol/L) calves but only tended (P = 0.10) to be higher in the treated Holsteins (301 and 417 nMol/L for control and vitamin D treatments, respectively). In Experiment II, three successive vitamin D dosage rates (2400, 4800, and 9600 IU/d; 14 d each) were administered to 14 Holstein steers (avg. BW = 150 kg) as above. Fecal samples were collected daily and serum samples weekly throughout the 42 d experimental period. No significant differences in fecal prevalence or serum vitamin D concentrations were observed for any of the vitamin D dosage rates or when data was combined across dosages. Differences in fecal shedding among the Holsteins and the beef calves in Experiment I are likely due to the difference in the vitamin D dose administered per unit of BW, as reflected in the serum concentrations of vitamin D. While no differences were observed in fecal shedding of E. coli O157:H7 in the second experiment, the concomitant lack of a treatment effect on serum concentrations of vitamin D suggest dose was a limiting factor. A third experiment using two collections (winter and summer) was conducted to determine if serum vitamin D concentrations are correlated to fecal shedding of E. coli O157:H7 in feedlot cattle. A fecal and blood sample were obtained from 60 randomly selected steers in each collection (n = 120 total fecal and 120 corresponding blood samples). As expected, season was highly correlated (r = 0.66) to serum vitamin D concentration with higher (P< 0.01) concentrations observed in the summer collection. Escherichia coli O157:H7 prevalence (percentage of positive samples) was not highly correlated (r = 0.16), although tended (P = 0.08) to be significant. The percentage of cattle shedding E. coli O157:H7 was 16.7 and 6.7 for the summer and winter collections, respectively. Taken together, these experiments tend to support the hypothesis that vitamin D is involved in the seasonal population dynamics of E. coli O157:H7 in cattle.